Colorectal Cancer is the third most common malignancy in the world, and represents approximately ten percent of the world's total cancer incidence [1]. Due to the aging world-wide population, CRC represents a serious public health problem requiring new actions that will minimize the impact of this disease. The chance of surviving CRC is closely related to the stage of the disease at diagnosis (as shown in Table 1; the earlier the diagnosis, the greater the likelihood of survival. For example, there is less than a 5% chance of 5-year survival when diagnosed late in the disease timeframe (Dukes' stage D), while there is greater than 90% chance of 5-year survival when diagnosed early (Dukes' stage A). Therefore, CRC patients would greatly benefit from early detection because of the effectiveness of surgical treatment early on.
Currently, the risk factors for CRC are not well understood. In fact, few specific risk factors other than diet have been established for the disease. Inflammatory bowel disease and familial adenomatous polyposis (FAP) increase risk, but still only account for a very small proportion of overall CRC incidence. Ethnic and racial differences, as well as migrant studies, suggests that environmental factors play a role in disease etiology, as incidence rates among migrants and their descendants climb rapidly, reaching those of the host country [2, 3]. Overall, fewer than 15% of CRC cases are familial, suggesting a large impact of diet, environment, and lifestyle on the etiology of the disease.
The most common current screening tests for CRC are: 1) the fecal occult blood test (FOBT), which is based on the assumption that cancers will bleed, and can therefore be detected in the stool using chemical or immunological assays; and 2) invasive methods that identify gross abnormalities. The FOBT is the most widespread test used for CRC, and involves a crude test for the peroxidase-like activity of heme in hemoglobin. However, the sensitivity of the test is only approximately 50%, with a 20% sensitivity for adenomas, due to the fact that not all adenomas and CRCs bleed [2].
Methods for identifying gross abnormalities can include flexible sigmoidoscopy and colonoscopy, as well as double-contrast barium enema and virtual colonoscopy. Colonoscopy is the next test for patients with a positive FOBT, and, with an 80% false positive rate, imposes unnecessary hazards and risks to a large number of individuals. Colonoscopy is usually the preferred method for screening average and increased-risk individuals over the age of 50 who have a history of CRC or prior adenomatous polyps, or other predisposing diseases such as inflammatory bowel disease. There is no evidence that screening using colonoscopy alone in average-risk populations reduces incidence or mortality [3], however, sigmoidoscopy and integrated evaluations comprising combinations of the above techniques can reduce the expected CRC rates in higher-risk individuals over a given length of time [4].
Although colonoscopy is still the standard test for the presence or absence of polyps and CRC, it can miss 15% of lesions >1 cm in diameter [5]. Complications with colonoscopy can include perforation, hemorrhage, respiratory depression, arrhythmias, and infection [6]. Approximately one in 1,000 patients suffer perforations and three in 1,000 experience hemorrhaging. Between one and three deaths out of 10,000 tests occur as a result of the procedure [3]. Other disadvantages such as the lack of trained personnel, patient discomfort, and high cost will likely prevent the colonoscopy from becoming a routine CRC screening method for the general population (see Table 2). Most sporadic CRCs are thought to develop from benign adenomas, of which only a small number will ever develop to malignancy. Given that the time period for malignant development from benign adenoma is five to ten years, the detection of adenomas across the general population by colonoscopy/sigmoidoscopy would require a gross overtreatment of patients, being both costly and potentially harmful [7].
Computerized Tomography Colonography (CTC), or virtual colonoscopy, is a recent non-invasive technique for imaging the colon, with reports varying dramatically on the performance characteristics of the assay (ranging between 39% and 94% specificity), due primarily to technological differences in the patient preparation and the hardware and software used for the analysis. Other limitations of CTC include high false-positive readings, inability to detect flat adenomas, no capacity to remove polyps, repetitive and cumulative radiation doses, and cost [6].
With advances in our understanding of the molecular pathology of CRC, several new screening methods based on DNA analysis from stool samples have emerged. These are typically PCR-based assays used to identify mutations known to occur in the adenoma-to-carcinoma sequence, or in familial CRC. Commonly screened gene mutations include KRAS, TP53, APC, as well as assays for microsatellite instability and hypermethylated DNA. Table 2, reproduced from Davies et al [7], compares current screening methods for CRC.
All of the methods described above are typically only capable of detecting CRC after the formation of an adenoma, and are generally not ideally suited for large-scale population screening. None of the above tests provide a quantitative assessment of a CRC-positive or negative promoting environment. Neither do any of the above tests provide a quantitative assessment of the effect of CRC on normal human biochemistry and related health states. Whether genomics-based tests will result in high diagnostic accuracy for sporadic CRC remains to be seen. Davies et at [7] outlined the features of an ideal screening test for CRC, as follows: 1) inexpensive; 2) simple to perform; 3) non-invasive; 4) represents the whole colon; 5) unambiguous interpretation of results (that is, high sensitivity, specificity, positive predictive value, and negative predictive value); 6) easy to teach; and 7) easy to maintain quality control.
A diagnostic assay based on small molecules or metabolites in serum fulfills the above criteria, as development of assays capable of detecting specific metabolites is relatively simple and cost effective per assay. The test would be minimally invasive and would be indicative of disease status regardless of colonic proximity. Translation of the method into a clinical assay compatible with current clinical chemistry laboratory hardware would be commercially acceptable and effective, and would result in rapid deployment worldwide. Furthermore, the requirement for highly trained personnel to perform and interpret the test would be eliminated.
CRC-specific biomarkers in human serum that could provide an assessment of CRC presence, of a CRC-promoting or inhibitory environment, of the physiological burden of CRC, or a combination of these characteristics would be extremely beneficial in the management of CRC risk, prevention, and treatment. A test designed to measure these biomarkers would be widely accepted by the general population as it would be minimally invasive and could possibly be used to monitor an individual's susceptibility to disease prior to resorting to, or in combination with, conventional screening methods.
Ovarian Cancer is the fifth leading cause of cancer death among women [8]. It has been estimated that over 22,000 new cases of ovarian cancer will be diagnosed this year, with 16,210 deaths predicted in the United States alone [9]. Ovarian cancer is typically not identified until the patient has reached stage III or IV and have a poor prognosis (5 year survival of around 25-30%) [10]. The current screening procedures for ovarian cancer involve the combination of bimanual pelvic examination, transvaginal ultrasonography and serum CA125 measurements [9]. The efficacy of this screening procedure for ovarian cancer is currently of unknown benefit, as there is a lack of evidence that the screen reduces mortality rates, and it is under scrutiny for the risks associated with false positive results [8, 11]. According to the American Cancer Society CA125 measurement and transvaginal ultrasonography are not reliable screening or diagnostic tests for ovarian cancer, and that the only current method available to make a definite diagnosis is surgically.
CA125, cancer antigen-125, is a high molecular weight mucin that has been found to be elevated in most ovarian cancer cells as compared to normal cells [9]. A CA125 test result that is higher than 30-35 U/ml is typically accepted as being at an elevated level [9]. There have been difficulties in establishing the accuracy, sensitivity and specificity of the CA125 screen for ovarian cancer due to the different thresholds to define elevated CA125, varying sizes of patient groups tested, and broad ranges in the age and ethnicity of patients [8]. According to the Johns Hopkins University pathology website the CA125 test only returns a true positive result for ovarian cancer in roughly 50% of stage I patients and about 80% in stage II, III and IV. Endometriosis, benign ovarian cysts, pelvic inflammatory disease and even the first trimester of a pregnancy have been reported to increase the serum levels of CA125 [11]. The National Institute of Health's website states that CA-125 is not an effective general screening test for ovarian cancer. They report that only about 3 out of 100 healthy women with elevated CA125 levels are actually found to have ovarian cancer, and about 20% of ovarian cancer diagnosed patients actually have elevated CA125 levels.
The identification of highly specific and sensitive ovarian cancer biomarkers in human serum, therefore, would be extremely beneficial, as the test would be non-invasive and could possibly be used to monitor individual susceptibility to disease prior to, or in combination with, conventional methods. A serum test is minimally invasive and would be accepted across the general population.